1. Field of the Invention
The present invention relates to a system for detecting a change in the pneumatic pressure of a tire on a motor vehicle.
2. Description of the Prior Art
Known apparatus and methods for detecting a change in the pneumatic pressure of a tire include a tire pressure detector having a pressure sensor attached to a wheel as disclosed in Japanese patent publication No. 56-10202, a method for detecting a reduction in the pneumatic pressure of a tire by comparing the detected speeds of wheels as disclosed in Japanese laid-open patent publication No. 63-305011, and a system for detecting a reduction in the pneumatic pressure of a tire by comparing the difference between wheel speeds and a motion condition of the motor vehicle as disclosed in Japanese patent application No. 4-46016.
In the system disclosed in Japanese patent application No. 4-46016, since any change in the wheel speeds which is caused by a reduction in the pneumatic pressure of the tire is of a very small value, it may be obscured or masked by errors contained in the wheel speeds depending on running conditions of the motor vehicle.
Therefore, highly accurate detection of any change which is brought about in the wheel speeds by a reduction in the pneumatic pressure of the tire requires that errors contained in the wheel speeds depending on running conditions of the motor vehicle be removed. One problem with such an approach is that if all running conditions which lead to large wheel speed errors are left out of consideration, then any pneumatic tire pressure reduction cannot be detected under those running conditions. There has been a demand for a system capable of detecting the pneumatic pressure of a tire under running conditions leading to large errors and upon a failure such as a brake failure which greatly affects running conditions.